John, I know that Grado makes some nice mono cartridges too. You might want to check into that. I just bought their 78C mono cartridge exclusively for playing 78 rpm records, and I'm very pleased with its performance and low cost. I believe they have models for playing micro-groove mono LPs, as well.
If you want to track the groove as intended, you need a true mono cartridge and probably a separate turntable system dedicated to that purpose. If you're just going to sum the stereo channels electronically, then you only need one tt setup and a preamp with a mono switch; but that's not quite the same thing, is it?
Best regards,
Frank :)